In association with the recent tendency for a higher degree of integration in semiconductor devices having multilayered wiring under 0.25 micron rule or below, as a space between the metal wiring layers becomes narrower, impedance between the metal wiring layers increases due to an electrostatic induction, and therefore there is a strong concern about delay in a response speed or increase in power consumption. To overcome this problem, it is necessary to make as low as possible a specific dielectric constant of an inter-layer insulating film provided between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer or between metal wiring layers.
The interlayer insulating film provided for the purpose as described above is generally formed on a semiconductor substrate by using a gas phase growth method such as the CVD method (i.e., Chemical Vapor Deposition Method) or a coating method such as the spin coating method.
In the silica-based coating film produced by using the latest techniques based on the CVD method, however, although it is possible to obtain a silica-based coating film with a specific dielectric constant of 3.0 or below, like in the coating films obtained by using the conventional coating method, there is still a defect that a film strength of the coating film becomes lower with decrease of the specific dielectric constant. In the CVD films made from polyaryl resin, fluorine-added polyimide resin, or fluorine resin, or those produced by using a coating film of any of the resins, although the dielectric constant is around 2, the adhesiveness to a surface of a substrate is rather low, and also the adhesiveness to a resist material used for fine mechanical processing is low. Furthermore there are the problems of, for instance, low chemical resistance or low resistance against oxygen plasma.
In the coating films obtained by using a coating liquid for forming a silica-based coating film, containing hydrolyte of alkoxysilane and/or halogenated silane, which has been widely used, although a silica-based coating film with a specific dielectric constant of 3.0 or below can be obtained, there are some problems relating to the low adhesiveness to a coated surface, the low film strength, the low chemical resistance, the low cracking resistance, the low resistance against oxygen plasma, and the like.
The present inventors made strenuous and concentrated efforts for solving the problems described above, and found that, by using a liquid composition containing a silicon compound obtained by hydrolyzing tetraalkyl orthosilicate (TAOS) and particular alkoxysilane (AS) in the presence of tetraalkyl ammonium hydroxide (TAAOH), it is possible to form an amorphous silica-based coating film with a low dielectric constant and having a high film strength, and also excellent in such properties as adhesiveness to a coated surface, flatness of a coated surface, moisture resistance (i.e., hydrophobicity), chemical resistance, cracking resistance, resistance against oxygen plasma, and workability by etching, and they filed patent applications based on the finding (Refer to patent document 1 and patent document 2). Furthermore, the present inventors found that, by coating the liquid composition on a substrate, heating the coated film under a temperature in the range from 80 to 350° C., and then curing the dried film at a temperature in the range from 350 to 450° C., it is possible to form an amorphous silica-based coating film with a low dielectric constant and having a lot of micropores therein, and they filed a patent application based on the finding (Refer to patent document 3).
Furthermore, the present inventors found that, by using a liquid composition containing a silicon compound obtained by hydrolyzing bis(trialkoxysillyl)alkane (BTASA) and alkoxysilane (AS) in the presence of tetraalkyl ammonium hydroxide (TAAOH), or a liquid composition containing a silicon compound obtained by hydrolyzing bis(trialkoxysillyl)alkane (BTASA), alkoxysilane (AS), and tetraalkyl orthosilicate (TAOS) in the presence of tetraalkyl ammonium hydroxide (TAAOH), it is possible to form an amorphous silica-based coating film with a low dielectric constant of 2.5 or below and having a high film strength, and also excellent in such properties as adhesiveness to a coated surface, flatness of a coated surface, moisture resistance (i.e., hydrophobicity), chemical resistance, cracking resistance, resistance against oxygen plasma, and workability by etching, and they filed a patent application on the finding (Refer to patent document 4).
Patent document 4 also discloses a method of forming an amorphous silica-based coating film with a low dielectric constant by coating the liquid composition on a substrate, heating the coated film at a temperature in the range from 80 to 350° C., and then curing the dried film at a temperature in the range from 350 to 450° C.
Patent document 5 discloses a method of forming a silica-based coating film by coating on a substrate a coating liquid containing polysilane obtained by reacting an organic compound having ROH group with a halogen-containing polysilane which is a product of a reaction between a monoorgano trihalosilane compound and a diorgano dihalosilane, drying the coated film at a temperature in the range from 50 to 250° C., and then curing the dried film at a temperature in the range from 300 to 500° C. in the presence of steam or oxygen. Although the silica-based coating film obtained by this method shows a specific dielectric constant of 3.0 or below, there is the problem that the silica-based coating film does not satisfy the requirements for physical properties such as a film strength or the like.
Furthermore, patent document 6 discloses a method of producing a porous silica by curing a porous silica precursor obtained from a solution containing a partially hydrolyzed condensate of an alkoxysilane and a surface active surfactant at a temperature in the range from 260 to 450° C. in an atmosphere containing H2O to remove organic compounds contained in the precursor. However, the porous silica film obtained by the method has mesopores therein and shows a low specific dielectric constant, but the film strength (as the modulus of elasticity) is disadvantageously low.    Patent document 1: JP 2004-149714 A    Patent document 2: JP 2006-117763 A    Patent document 3: JP 2004-153147 A    Patent document 4: Japanese Patent Application No. 2005-371017    Patent document 5: JP H11-256106 A    Patent document 6: JP 2005-116830 A